The natural history of Str. pneumoniae infections is being studied in a group of infants now 3-4 years of age, who have been followed since birth. The relation between pneumococcal carriage, infection and immunity is being determined in prospective fashion. Pneumococci are serotyped with Danish reference antisera. Antibody is determined in this laboratory by NBT assay and by a newly developed ELISA method. Collaborative arrangements with Dr. Gerald Schiffman provide for antibody assay by the RIA technique. In addition to patients prospectively investigated, seroepidemiologic studies are being performed, utilizing sera for children and adolescents stored from prior investigations. Included in the latter are sera from patients with well-defined pneumococcal infections. including otitis media and bacteremia. Comparative epidemiology of pneumococcal infections in children and adults is investigated by examining collections of clinically relevant pneumococci from pediatric and adult infectious disease services. Host age range, clinical syndrome, serotype causing disease, serological response to infections and seasonal trends of pneumococcal disease are being determined.